Using the Nernst equation, calculate the cell potential for a two-electron transfer (n = 2) with E° = 1.10 V when Q = 1.0×10^-3.

• A. 1.10 V
• B. 1.18 V
• C. 1.18874 V
• D. 0.99 V

Answer: (C)

The Nernst equation shows how the cell potential shifts from the standard potential when the reaction quotient Q isn’t 1. At 25°C, E = E° − (0.05916/n) log Q. With n = 2, E° = 1.10 V, and Q = 1.0×10^-3, log Q = log(10^-3) = −3. So E = 1.10 − (0.05916/2)(−3) = 1.10 + 0.08874 = 1.18874 V. Since Q is less than 1, the logarithm is negative and its negative sign in the equation adds a positive term, pushing E above E°. The calculated cell potential is 1.18874 V.